DXF: Handle linetype scaling

Description

Currently the DXF driver does not handle linetype scaling at all. Linetypes may be scaled by the scale factor specified on the entity (group code 48) or in the global $LTSCALE variable.

This patch implements linetype scaling in the reading-in side of the DXF driver.

I haven't touched the writer, and it will need a lot of reworking if it is to handle linetype scaling properly. The incorrect round-tripping of DXF files with linetype scales is not new, but I have added a comment to drv_dxf.html to make users aware of this caveat.